1. Field of the Invention
The present invention relates to systems and methods for controlling a starting and the subsequent vehicle acceleration procedure of a motor vehicle.
2. Description of the Background Art
JP-A 08-207605 discloses a motor vehicle operable in 4WD mode or in 2WD mode. The vehicle can start in 4WD mode. A transfer has a torque distribution clutch. Regulating hydraulic fluid pressure applied to the clutch in response to a control signal can alter amount of torque branched off output torque of an engine. A controller generates the control signal. The controller uses, as inputs, accelerator pedal opening and wheel speeds. The controller calculates acceleration slip based on the wheel speeds. The controller calculates a first value of torque based on the accelerator pedal opening and a second value of torque based on the acceleration slip. The controller establishes a threshold vehicle speed (20 km/h, for example) and compares vehicle speed to the threshold vehicle speed. The controller sets a greater one of the first and second values as a base torque when the vehicle speed is lower than the threshold vehicle speed. The controller sets the second value as the base torque when the vehicle speed is equal to or higher than the threshold vehicle speed. The controller takes vehicle deceleration into account in determining the control signal based on the base torque to allow rapid drop in torque transmitting capacity of the clutch, thereby to quickly disable 4WD. This measure is effective in avoiding occurrence of tight corner braking.
In a starting and the subsequent vehicle acceleration procedure of this known system, since there is always a time delay, disabling a 4WD mode when the vehicle speed reaches the threshold vehicle speed causes acceleration slip to occur until another 4WD mode is enabled. This procedure is disadvantageous in that the acceleration slip during the transition is tamed by a threshold vehicle speed only, so that the threshold vehicle speed could not be lowered below a lower limit even if required by demand for enhanced fuel economy.